Cobaloximes: selective nitrite reduction catalysts for tandem ammonia synthesis

Herein, we demonstrate cobaloximes as a bioinspired molecular platform for exclusive ammonia synthesis via electrocatalytic NO2- reduction (eNO(2)(-)RR), which attained 98.5% faradaic efficiency (FE) under close-to-neutral conditions. Mechanistic studies illustrated that cobaloximes furnished effective binding with NO2- and nitrogenous intermediates, along with continuous, rapid 6e(-)/8H(+) transfer with an intramolecular hydrogen bonding framework. As the cobaloxime skeleton displayed tunable structures on axial and equatorial sites for selective NO2- to NH4+ transformation, integrating cobaloximes with multi-walled carbon nanotubes (MWCNTs) as working electrodes attained advantageous ammonia yields of 19.3 mg h(-1) mg(Cat)(-1) with >95% FE at -0.5 V vs. RHE. More strikingly, cobaloxime-catalyzed NO2- to NH4+ transformation was coupled with plasma-driven N-2 oxidation (pNOR) to convert ambient air into NH4Cl at a mmol-scale. This work demonstrates promising prospects of bioinspired molecular catalytic platforms for effective and selective ammonia and nitrogenous chemical synthesis via NOx-.

Automated summary

In this study, cobaloximes are demonstrated as a bioinspired molecular platform for ammonia synthesis through electrocatalytic reduction of NO2-. The cobaloximes achieved 98.5% faradaic efficiency under close-to-neutral conditions. Mechanistic studies showed that cobaloximes effectively bind with NO2- and nitrogenous intermediates and facilitate rapid electron and proton transfer. The cobaloxime catalyst, when integrated with multi-walled carbon nanotubes as working electrodes, exhibited advantageous ammonia yields and high efficiency. Furthermore, the cobaloxime-catalyzed NO2- to NH4+ transformation, when coupled with plasma-driven N-2 oxidation, enabled the conversion of ambient air into NH4Cl at a mmol-scale.